Intervertebral disc replacement

ABSTRACT

According to some embodiments of the invention, an intervertebral disc replacement includes a first layer having a lower surface for contacting a first vertebral bone, a second layer coupled to the first layer, the second layer comprising a plurality of compressible column springs, and a third layer coupled to the second layer, the third layer having an upper surface for contacting a second vertebral bone. Each of the plurality of compressible column springs comprises a plurality of stacked coils, and each of the plurality of stacked coils has a spring constant (K). At least one of the plurality of compressible column springs includes a first coil having a first spring constant and a second coil comprising a second spring constant, wherein the first spring constant is different from the second spring constant.

This application claims priority to U.S. Provisional Application No.62/259,540 filed Nov. 24, 2015, the entire content of which is herebyincorporated by reference.

BACKGROUND

1. Technical Field

The field of the currently claimed embodiments of this invention relatesto intervertebral disc replacements, and methods of producingintervertebral disc replacements.

2. Discussion of Related Art

The spine plays a prominent role in spinal cord and nerve rootprotection, weight bearing, and movement. The intervertebral disc is acore component of the spine and the spinal motion axis. Theintervertebral disc acts in the spine as a stabilizer, and as amechanism for force distribution between the vertebral bodies. Withoutthe disc, collapse of the intervertebral space occurs in conjunctionwith abnormal joint mechanics and premature development of arthriticchanges. However, existing intervertebral disc replacements often reducecompressive support, overload same level facet joints, and generateexcessive wear particles from the bearing surfaces. Accordingly, thereis a need for improved intervertebral disc replacements.

SUMMARY

According to some embodiments of the invention, an intervertebral discreplacement includes a first layer having a lower surface for contactinga first vertebral bone, a second layer coupled to the first layer, thesecond layer comprising a plurality of compressible column springs, anda third layer coupled to the second layer, the third layer having anupper surface for contacting a second vertebral bone. Each of theplurality of compressible column springs includes a plurality of stackedcoils, and each of the plurality of stacked coils has a spring constant(K). At least one of the plurality of compressible column springsincludes a first coil having a first spring constant and a second coilcomprising a second spring constant, wherein the first spring constantis different from the second spring constant.

According to some embodiments, at least a second compressible columnspring of the plurality of compressible column springs includes a coilhaving a third spring constant, wherein the third spring constant isdifferent from at least one of the first and second spring constants.The disc replacement undergoes compression at a first rate when acompressive force is applied until a height of the disc replacementreaches a first predetermined value, and undergoes compression at asecond rate as the height of the disc replacement is further reduced,wherein the second rate is less than the first rate.

According to some embodiments, each of the first layer and the thirdlayer has a plurality of fixation mechanisms that engage the firstvertebral bone or the second vertebral bone to fix the position of theintervertebral disc replacement with respect to the first and secondvertebral bones. Each coil has a cross-section, and the cross-section ofeach coil has a flat lower surface and a flat upper surface that isparallel to the flat lower surface. Each cross-section has a maximumwidth, and according to some embodiments, each of the flat upper surfaceand the flat lower surface has a width that is at least 30% of themaximum width. According to some embodiments, each of the flat uppersurface and the flat lower surface has a width that is at least 90% ofthe maximum width. According to some embodiments, each cross-section issubstantially rectangular.

According to some embodiments, the plurality of compressible columnsprings are arranged in one or more concentric rings around the centerof motion of the intervertebral disc replacement. At least one of theplurality of compressible column springs includes coils that are woundclockwise, and at least one of the plurality of compressible columnsprings includes coils that are wound counter-clockwise. Theintervertebral disc replacement mimics the compressive, extension,flexion, and rotational behavior of a human intervertebral disc.

According to some embodiments, the intervertebral disc replacementfurther includes a barrier attached to the first layer and the thirdlayer and enclosing the second layer, wherein the barrier forms afluid-tight seal with the first layer and the third layer.

According to some embodiments, each column spring has an outercircumference between about 3 mm and about 40 mm. According to someembodiments, the intervertebral disc replacement includes one or more oftitanium, nitinol, cobalt chrome, and high density polycarbonate.According to some embodiments, each of the plurality of compressiblecolumn springs has a height that decreases when a compressive force isapplied, and a structure that prevents the height from decreasing beyonda predetermined limit.

According to some embodiments, a difference between the first springconstant and the second spring constant is determined by a difference inat least one of a group consisting of a height of a cross section, awidth of a cross-section, a pitch, and a material of the first coil andthe second coil. According to some embodiments, the first layer, thesecond layer, and the third layer are one unitary piece.

According to some embodiments, the second layer includes a firstplurality of compressible column springs attached to the upper surfaceof the first layer, a second plurality of compressible column springsattached to the lower surface of the third layer, and a fourth layerdisposed between and attached to the first plurality of compressiblecolumn springs and the second plurality of compressible column springs.According to some embodiments, one or both of the first layer and thethird layer has a height that varies.

According to some embodiments of the invention, a method of producing anintervertebral disc replacement includes forming, using a powder-based3D printer, a first layer. The method further includes forming, usingthe powder-based 3D printer, a second layer on top of the first layer,the second layer comprising a plurality of compressible column springs.The method further includes forming, using the powder-based 3D printer,a third layer on top of the second layer, and removing unbound powderfrom the plurality of compressible column springs. Each of the pluralityof compressible column springs includes a plurality of stacked coils,and each coil of each of the plurality of stacked coils has a springconstant. At least one of the plurality of compressible column springsis formed to include a first coil having a first spring constant and asecond coil having a second spring constant, wherein the first springconstant is different from the second spring constant.

According to some embodiments, the method further includes, prior toforming the intervertebral disc replacement, obtaining at least one ofx-ray, magnetic resonance imaging (MRI), computed tomography (CT),patient body mass above the disc replacement, and determining the anglesof the end plates and the position of each of the plurality ofcompressible column springs and the spring constants of each of theplurality of stacked coils based on the obtained data to match a patientspecific level lordosis and movement needs.

According to some embodiments, forming the second layer includesforming, using the powder-based 3D printer, a first plurality ofcompressible column springs attached to the upper surface of the firstlayer, forming, using the powder-based 3D printer, a fourth layerattached to the first plurality of compressible column springs, andforming, using the powder-based 3D printer, a second plurality ofcompressible column springs attached to the fourth layer and the lowersurface of the third layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objectives and advantages will become apparent from aconsideration of the description, drawings, and examples.

FIG. 1 shows a disc replacement according to some embodiments of theinvention;

FIG. 2 shows the first and second layers of an intervertebral discreplacement according to some embodiments;

FIG. 3 shows a cross section of an intervertebral disc replacement;

FIG. 4 illustrates the compression characteristics of the intervertebraldisc replacement according to some embodiments;

FIG. 5 shows a plot demonstrating the dependence of the compression rateon the applied load and two spring constants of one or more columnsprings;

FIG. 6 shows an intervertebral disc replacement with beveled coilsaccording to some additional embodiments;

FIG. 7 shows the first and second layers of the disc replacement of FIG.6;

FIG. 8 shows a disc replacement wherein the coils of the column springsare maximally compressed;

FIG. 9 shows a cross-section of the disc replacement shown in FIG. 8;

FIG. 10 shows the first and second layers of the disc replacement shownin FIGS. 8 and 9;

FIG. 11 shows additional images of a fully-compressed disc replacement;

FIG. 12 shows a disc replacement including a barrier surrounding thesecond layer;

FIG. 13 shows an intervertebral disc replacement comprising two layersof column springs according to some embodiments of the invention;

FIG. 14 shows the intervertebral disc replacement of FIG. 13 wherein thetop plate has been rendered transparent;

FIG. 15 shows a side view of the disc replacement shown in FIGS. 13 and14;

FIG. 16 shows a disc replacement in which the clockwise- andcounter-clockwise-wound coils are alternated;

FIG. 17 illustrates a disc replacement having a first plurality ofcolumn springs below a second plurality of column springs;

FIG. 18 shows a single compressible column spring according to someembodiments;

FIG. 19 shows an intervertebral disc replacement according to someadditional embodiments, wherein the second layer comprising acompressible lattice;

FIG. 20 shows a zoomed-in image of the compressible lattice according tosome embodiments;

FIG. 21 shows a side view of an intervertebral disc replacementcomprising a compressible lattice;

FIG. 22 shows a cross-section of an intervertebral disc replacementcomprising a compressible lattice;

FIG. 23 shows the first and second layers of an intervertebral discreplacement comprising a compressible lattice; and

FIG. 24 shows a top view of the compressible lattice.

DETAILED DESCRIPTION

Some embodiments of the current invention are discussed in detail below.In describing embodiments, specific terminology is employed for the sakeof clarity. However, the invention is not intended to be limited to thespecific terminology so selected. A person skilled in the relevant artwill recognize that other equivalent components can be employed andother methods developed without departing from the broad concepts of thecurrent invention. All references cited anywhere in this specification,including the Background and Detailed Description sections, areincorporated by reference as if each had been individually incorporated.

The human intervertebral disc is composed of an inner gel-like matrixcalled the nucleus pulposus and an outer surrounding fibrous band calledthe annulus fibrosus. The disc's viscoelastic attributes provide shockabsorbing capability that attenuate and transmit compressive and shearstresses. This force dampening protects the surrounding soft tissues andosseous structures from injurious loads. Furthermore, the disc'sinterposed position between adjacent vertebral bodies contributes to thespine's flexibility and provides mechanical restraints to excessive anddeleterious spinal motion. In conjunction, the intervertebral disc, thevertebral bodies, and the facet joints form the “triple joint complex”that allows for physiologic spinal flexion, extension, lateral bendingand rotation.

Chronic neck and low back pain are prevalent and costly diseases.Yearly, 15-20% of adults report at least one episode of neck pain, andclose to half of these individuals seek care. Lifetime incidence oflumbar back pain (LBP) is reported to be 60-90% with annual incidence of5%. Each year, 14.3% of new patient visits to primary care physiciansare for LBP, and nearly 13 million physician visits are related tocomplaints of chronic LBP, according to the National Center for HealthStatistics.

Annually, it is estimated that 11-14% of workers will experience somelimitation in their work activities due to neck pain. In 1990, 400,000industrial low back injuries resulted in disability in the UnitedStates. This number represents approximately 21% of injuries in theworkplace and accounted for 31% of compensation payments.

A common cause of neck and lower back pain is a compromisedintervertebral disc. Through various mechanisms, such as trauma, aging,or other disease processes, the intervertebral disc can become damaged.This damage results in some degree of pathologic change to the nucleuspulposus and the annulus fibrosis. Frequently, the nucleus pulposusbecomes desiccated resulting in diminished load dampening properties anddisc elasticity. This in turn leads to increased stresses on the annulusand facet joints, giving rise to the tearing of the annulus fibers andinjury to facet cartilage. Further deterioration of disc components maylead to circumferential bulging of the disc, disc herniation, and facetjoint arthritis. Pain receptors sense this deterioration and stimulate apain response commonly referred to as axial neck or back pain. Painprimarily emanating from the disc injury is referred to as discogenicpain. Moreover, an extruded disc through direct compression ofsurrounding nerve roots/spinal cord and/or an inflammatory modulatedmechanism can cause radicular symptoms (nerve pain) in the upper and/orlower extremities. Clinically, this underlying pathology presents assome combination of motor weakness, pain, and sensory dysthesia.

Commonly, surgeons perform a spinal arthrodesis to address the triplejoint dysfunction and pain generated by the compromised intervertebraldisc. During an arthrodesis the residual intervertebral disc is excisedand an osteoconductive spacer is implanted and secured between theneighboring vertebrae by some combination of screws and plates/rodsattached to the vertebra. With time the immobilized spacer forms a bonebridge, fusing adjacent vertebral bodies together. This procedure hasprovided for significant pain relief for many patients, but to thedetriment of spinal motion near the fusion segment. Removal of motion atone segment alters the biomechanical environment, and necessarilymagnifies the stresses at the triple joint complexes of adjacent joints.This adjacent joint mechanical overload accelerates deterioration,catalyzing the same pain generating process that arthrodesis wasintended to resolve. This epiphenomenon is clinically described asadjacent joint disease. Therefore, with the passing of time, multilevelfusions are often indicated to treat the initial arthrodesis.Furthermore, arthrodesis is not without potential surgicalcomplications, such as nonunion, infection, and injury to major bloodvessels manipulated during the surgical approach.

To address the limitations of spinal arthrodesis, spinal intervertebraldisc arthroplasty has gained popularity and has proven to be a viablealternative to arthrodesis for the treatment of discogenic spinaldisease. Similar to arthrodesis, the injured intervertebral disc isexcised, but instead of fusion of adjacent vertebra, a prosthetic discis implanted that allows for preservation of motion. Many devices arecurrently marketed, both in the cervical and lumbar spine. Differentdevices can be primarily distinguished by their degree of constraint,type of bearing surfaces, geometry of bearing surfaces, and end plateinterface.

Although current devices have demonstrated some success, overall resultshave left many practitioners and patients disappointed, especially withregards to same level and adjacent level facet disease. Such devicespotentially suffer from various types of problems such as excessiveun-constrained motion, distraction of disc space, inappropriate axes ofrotation, wear debris, inadequate compression and shear stressattenuation, and surgical implantation and/or removal complications andrisks. One type of existing replacement disc employs a ball-and-socketstructure to allow some relative motion of the adjacent vertebrae.However, this disc design does not have a compressive component, and istherefore not able to act as a shock absorber. In addition, the bearingsurface experiences significant friction due to the large load exertedon the disc. This friction can lead to pieces of the bearing surface andsurrounding socket being sheared off, introducing loose debris into thearea surrounding the disc. The loose debris can result in osteolysis(bone loss) and chronic inflammation at the implant site. Accordingly,opportunities exist for improvement on current disc replacementtechnology.

The disc replacement device described herein affords desirablecharacteristics and provides solutions to clinically relevantdeficiencies in current spinal disc arthroplasty implants. The basicfunctional unit can be arranged in a unique geometric pattern thatallows for spring-like mechanical properties. The unit can be composedof titanium and/or other biocompatible materials. This design stands instark contrast to the “ball and socket” design of the vast majority ofcurrently marketed products, and offers clear advantages to currenttechnology.

First, a spring-like device in accordance with the principles of theinvention demonstrates similar biomechanical properties to theviscoelastic properties of the native intervertebral disc. Thus, thedevice design can be readily manipulated to share more of the motionsegment load, thereby achieving desirable compression and shear stressdampening. Clinically, this can lead to decreased incidence of facet andadjacent level disc disease, the preponderant reasons for current devicefailures. The disc can include a plurality of compressible springs. Eachspring can be made of a plurality of coils, and the coils that make up asingle spring may have different spring constants, such that the springconstant along the length of a single spring changes. Having a number ofsuch springs in the disc replacement allows the disc replacement to havea different response to low compression loads compared to highcompression loads, mimicking the behavior of a native disc.

In addition, the disc includes springs that are wound clockwise andsprings that are wound counter-clockwise. By virtue of the clockwise orcounter-clockwise winding of the springs, each spring can act as anantagonist to a strategically placed oppositely-wound spring. Thisallows for some rotation of the disc, but modulates unconstrainedrotation. Thus, the clockwise and counter-clockwise springs arepositioned to enable the disc to provide rotational characteristicssimilar to those of a native disc.

Second, the implant can be engineered to more precisely achieve thetargeted instantaneous axes of rotation and spinal motion parameters.Accordingly, the load experienced by surrounding joints can beminimized, again reducing the likelihood of same level facet disease andadjacent level disc disease. As described above, the disc replacementcan have a plurality of springs that can be finely tuned to mimic thecompressive characteristics of a native disc. For example, certain areasof the plurality of springs can be designed to be more or less resistantto compression, by changing the spring constant of individual or evenportions of springs. One can thus design regions of the disc known toexperience greater forces to be less compliant, while less load-bearingareas can be more compressible.

The compressive lattice can also be designed to mimic the instantaneousaxes of rotation that are characteristic of a native disc in its usualplanes of motion. For motion at a specific spinal segment, one canimagine the superior vertebra rotating about a fixed point in the disc.However, since the superior and inferior vertebra can move relative toone another, this point within the disc can change. Thus, the terminstantaneous axis of rotation is used to describe the axis of rotationat a particular time in the motion arc. The device in accordance withthe principles of the invention can be designed in such a manner as toplace the axis of rotation in an appropriate position, so the resultingmoment can place lesser loads on the facet joints, leading to diminishedpain and lessening the likelihood of clinical failure.

The disc replacement described herein can be designed to have one ormore axes of rotation in the same anatomic region as a nativeintervertebral disc. For example, the column springs can be morecompressible in certain areas to facilitate rotation at appropriatepositions, and can inhibit rotation at inappropriate positions. Thecompressive and rotational qualities can also be fine-tuned depending onthe intended anatomic position of the replacement disc. Cervicalintervertebral discs and lumbar intervertebral discs can be designeddifferently to meet the particular motion profile, instantaneous axis ofrotation characteristics, and load-bearing requirements of the targetregion of the spine.

The disc replacement has flexion, extension, and compression propertiessimilar to those of a native disc. For example, when a patient with thedisc replacement implanted in their spine bends forward, the anteriorends of the upper and lower layers of the disc move toward each other,while the posterior ends of the upper and lower layers spread apart fromeach, as is the case in a native disc. Similarly, during backward andlateral flexion, the upper and lower layers tilt with respect to eachother in a manner that provides flexibility to the patient, but stillprovides shock-absorbing qualities that protect the adjacent vertebra.

Third, the device affords a near frictionless bearing surface, asopposed to the metal-polyethylene or metal-metal bearing surface ofcurrent implants. The design yields significantly less wear debris andminimizes the risk of osteolysis and chronic inflammation at the implantsite. This also results in a decrease in the incidence of implantloosening, and a reduction in revision rates due to implant loosening.

Fourth, the disc replacement may be manufactured using a 3D printer,allowing it to be constructed to meet the exact, unique characteristicsrequired for a given patient. The flexibility to augment implantdimensions and configurations via the 3D printing manufacturing processenables the device to address pre-operative sagittal plane deformity andreestablish normal lordosis. The overall dimensions of the discreplacement, including its height and width, as well as the shape of thebone-contacting surfaces, can be customized. In addition, the individualpieces can be structured to provide a compressive response like that ofa native disc.

Finally, the intrinsic mechanical properties of the disc replacementprovide rotational stability minimizing the likelihood of coronal planedeformity due to excessive rotational motion. As described above, thedisc replacement is designed to promote desired rotation, and to limitor prohibit undesired rotation. Thus, the disc replacement can allow forrotational flexibility while preventing excessive rotational motion andthe complications associated therewith.

Some embodiments of the current invention comprise a three-layerintervertebral disc replacement that allows for compression, bending,and rotation. FIG. 1 shows a disc replacement 100 that includes a firstlayer 102 having a lower surface for contacting a first vertebral bone.The disc replacement 100 includes a second layer 104 coupled to thefirst layer 102, the second layer 104 comprising a plurality ofcompressible column springs 106, 108. The disc replacement 100 includesa third layer 110 coupled to the second layer 104, the third layer 110having an upper surface for contacting a second vertebral bone. Each ofthe plurality of compressible column springs includes a plurality ofstacked coils, wherein each coil of each of the plurality of stackedcoils has a spring constant (K). At least one of the plurality ofcompressible column springs includes a first coil 112 having a firstspring constant and a second coil 114 having a second spring constant,wherein the first spring constant is different from the second springconstant.

The term “layer” is intended to have a broad meaning that should not belimited to a particular construct and/or embodiment. The term “layer” isgenerally used to refer to a region, area, portion, or section of thedisc replacement.

The natural intervertebral disc is a mechanical entity with viscoelasticproperties whose attributes are correlated to the speed by which forceis exerted on it. In order to mimic this property with a compressiblecolumn spring, the compressible column spring is designed to havedifferent K's along its length. This allows the disc replacement torespond differently to sudden impact versus gradually applied loads. Thedifference between resistance to impact and resistance to graduallybuilt pressure is important to keep the vertebrae and the spinal columnsafe under impact while allowing freedom of movement under gradualapplication of pressure.

According to some embodiments of the invention, the intervertebral discreplacement has at least a second compressible column spring of theplurality of compressible column springs that includes a coil 116 havinga third spring constant, wherein the third spring constant is differentfrom at least one of the first and second spring constants. Because thespring constant of the second layer 104 can vary from column spring tocolumn spring and also with a single column spring, the compressiveresponse of the disc replacement can be finely tuned over each cubicmillimeter of the disc replacement.

According to some embodiments of the invention, the first layer 102 andthird layer 110 have a thickness that varies such that the upper surfaceand lower surface of a single layer may not be parallel. This design ofthe layers provides patient specific lordosis and kyphosis. The discreplacement be asymmetric. The thickness of the layers themselves maynot be constant, and the distance between the upper surface of the firstlayer and the lower surface of the third layer may vary. Further, thedistance between the lower surface of the first layer and the uppersurface of the third layer may also vary. This allows the discreplacement to have a specific geometric shape that fits the space thatthe natural disc once occupied. The design also allows forpatient-specific as well as level specific range of motion with as closeto natural as possible free movement and resistance to movement.

There are a variety of ways in which the spring constant of theplurality of coils can be varied. For example, the outer radius of thecoils can be varied, as well as the inner radius. Thicker coils(thickness=outer radius−inner radius) will generally have a higherspring constant, while thinner coils will generally have a lower springconstant. The height of a cross-section of a coil can be varied, as wellas the pitch of the coils that make up the column springs. The pitch isdefined as the distance in between a column spring's adjacent coils. Thespring constant can also be varied by varying the materials used to makethe spring. For example, if one coil of the spring is made from a firstmaterial and another coil of the spring is made from a second materialhaving different elastic and deformation properties, the two coils willhave different spring constants even if their dimensions are the same.

FIG. 2 shows the first and second layers of an intervertebral discreplacement according to some embodiments. In some embodiments, theplurality of compressible column springs are arranged in one or moreconcentric rings around the center of motion of the intervertebral discreplacement. This configuration can provide stability at the center ofmotion, while allowing the disc replacement to tilt in response toforces applied at the edge of the disk. According to some embodiments,the spring or springs closer to the center of motion may have a higherspring constant than the springs closer to the outer edges of the discreplacement. The embodiments of the invention are not limited to thisconfiguration, however, and other strategic pattern can be employed toalter the compliance of the entire disc replacement. According to someembodiments of the invention, the disc replacement is designed tomaximize the number of column springs in the second layer, withoutallowing any two column springs to come into contact with each otherwhen the disc replacement is compressed or rotated.

FIG. 3 shows a cross section of an intervertebral disc replacement. Asshown in FIG. 3, each coil has a cross section 300. According to someembodiments, the cross-section 300 of each coil has a substantially flatlower surface 302 and a substantially flat upper surface 304 that issubstantially parallel to the substantially flat lower surface 302. Thecross sections can be substantially rectangular. Each cross-section hasa width 306 and a height 308. According to some embodiments, each columnspring has an outer diameter 310 that is between about 3 mm and about 40mm. The disc replacement can also have one or more fixation mechanisms312 that engage the adjacent vertebra to fix the position of the discreplacement with respect to the vertebra. The fixation mechanisms can bespikes with tapered ends that contact the vertebra, though theembodiments of the invention are not limited to this design.

FIG. 4 illustrates the compression characteristics of the intervertebraldisc replacement according to some embodiments. The left-hand portion ofFIG. 4 is a schematic illustration of a column spring having three uppercoils with a first spring constant K1, and three lower coils with asecond spring constant K2 that is greater than the first spring constantK1. When a compressive force is applied to the spring, the spring firstcompresses at a first rate determined primarily by the spring constantK1 of the upper springs. The lower springs undergo little or nocompression. However, once the upper springs are mostly or fullycompressed, the spring then compresses at a second rate that isdetermined primarily by the second spring constant, K2. The lower coilscompress at this second rate until they, too, are fully compressed.Accordingly, the disc replacement undergoes compression at a first ratewhen a compressive force is applied until a height of the discreplacement reaches a first predetermined value, and then the discreplacement undergoes compression at a second rate as the height of thedisc replacement is further reduced, wherein the second rate is lessthan the first rate. The plurality of springs by virtue of their coildesign will impart a different compliance to the disc replacement at lowloads compared to high loads. At low loads and slower rate of loadapplication the disc replacement will be compliant with low stiffness,and for high loads or rapid rates of load application the discreplacement will be stiff or non-compliant.

The dependence of the compression rate on the applied load and twospring constants is illustrated in the plot in FIG. 5. Having two ormore coils with different spring constants within a single column springenables to the disc replacement to respond differently to static loadsversus dynamic loads, enabling the intervertebral disc replacement tomimic the compressive, extension, flexion, and rotational behavior of ahuman intervertebral disc.

While FIGS. 4 and 5 illustrate a column spring with coils having twodifferent spring constants, the embodiments of the invention are notlimited to column springs with coils with just two spring constants. Acolumn spring may have coils of three or more spring constants. Coilshaving the same spring constant may be grouped together within thecolumn spring, or may be interspersed throughout the column spring.Further, when a compressive force is applied to a spring, coils withinthe spring having different spring constants may undergo compression atthe same time, but at different rates.

Some column springs may only have coils of one spring constant, suchthat the spring constant along the full length of the column springremains unchanged. Further, a disc replacement according to someembodiments may comprise a single column spring. The coils of the singlecolumn spring may have one or more spring constants. A disc replacementaccording to some embodiments may include a plurality of column springs,each having a single spring constant along its length. The springconstants of the plurality of column springs may all be the same, or mayvary from column spring to column spring.

FIG. 6 shows an intervertebral disc replacement with beveled coilsaccording to some additional embodiments, and FIG. 7 shows the first andsecond layers of the disc replacement of FIG. 6. As shown in FIGS. 6 and7, the coils of the column springs can have edges that are beveled orrounded. The coils have a cross-section with a flat upper surface and aflat lower surface, but the cross-section is not a rectangle like thecross-section 300 in FIG. 3. The cross-section has a maximum width, andeach of the flat upper surface and the flat lower surface has a widththat is at least 30% of the maximum width. According to some embodimentsof the invention, each of the flat upper surface and the flat lowersurface has a width that is at least 90% of the maximum width.

The flat upper and lower surfaces of the cross-sections of the coilscreates a large surface area over which pressure can be distributed. Ifthe coils are sufficiently compressed that they come into contact witheach other, the forces applied by one coil on an adjacent coil arespread out over the flat surface, reducing the pressure at any singlepoint. If the coils had a circular cross-section, each coil wouldcontact its neighbor at only the top or bottom point of thecross-section, concentrating the force on that point. This potentiallyleads to wear and the release of debris as pieces of the spring breakoff due to excessive force. The design of the coils according to someembodiments of the invention prevents erosion of the coils bydistributing forces over a larger surface area. When the column springis uncompressed or partially compressed, a space exists between adjacentcoils of a given spring, allowing the disc replacement to absorb shockexerted on the spine.

FIGS. 8-10 shows an intervertebral disc replacement undergoing completecompression. FIG. 8 shows how the coils of the column springs aremaximally compressed. Each coil, with the exclusion of the upper-mostand lower-most coils, comes into contact with the coils above and belowit such that the distance between the first layer and the third layerreaches a minimum value, beyond which the disc replacement can no longerbe compressed. The intervertebral disc replacement can thus be designedto have a minimum height that is predetermined based on the patient'sphysiology.

FIG. 9 shows a cross-section of the disc replacement shown in FIG. 8.The cross-sections of the coils are visible, showing how the flat uppersurface of each coil, with the exclusion of the upper-most coils,contacts the flat lower surface of the coil above it. This distributesthe forces on each coil over a larger area, thereby reducing thepressure at any single point. FIG. 10 shows the first and second layersof the disc replacement shown in FIGS. 8 and 9. While FIG. 10 shows thatall the column springs become maximally compressed at the same height,the embodiments of the invention are not limited to this concept. Theminimum height of the disc replacement may be determined by themaximally-compressed height of one or more of the springs. Further, theminimum height might be different for different regions of the discreplacement. For example, the middle of the disc replacement may becompressible to a first minimum height, while the outer edges of thedisc replacement may be further compressed to a second minimum height.FIG. 11 shows additional images of a fully-compressed disc replacement.

According to some embodiments of the invention, the intervertebral discreplacement includes a barrier disposed between the first layer and thethird layer and surrounding the second layer. An example of a discreplacement including a barrier is shown in FIG. 12. The barrier 1200surrounds the second layer, preventing the body from protruding orgrowing into the area of the column springs, thus protecting the springsfrom interference by the body. The barrier may be made of one or more ofGortex, polycarbonate, silastic, and any biocompatible membrane ortissue. The barrier may be made of a material that is impermeable andwear resistant. The barrier may be mechanically or chemically bonded tothe upper and lower layers, or both mechanically and chemically bonded.The barrier may form a fluid-tight seal with the first layer and thethird layer.

An intervertebral disc replacement comprising two layers of columnsprings according to some embodiments of the invention is shown in FIG.13. The second layer includes a first plurality of compressible columnsprings 1300 attached to the upper surface of the first layer, a secondplurality of compressible column springs 1302 attached to the lowersurface of the third layer, and a fourth layer 1304 disposed between andattached to the first plurality of compressible column springs 1300 andthe second plurality of compressible column springs 1302. FIG. 14 showsthe intervertebral disc replacement of FIG. 13 wherein the top plate hasbeen rendered transparent. FIG. 15 shows a side view of the discreplacement shown in FIGS. 13 and 14.

According to some embodiments of the invention, at least one of theplurality of compressible column springs in the intervertebral discreplacement includes coils that are wound clockwise, and at least one ofthe plurality of compressible column springs includes coils that arewound counter-clockwise, as shown in FIG. 1. FIG. 16 shows a discreplacement in which the clockwise- and counter-clockwise-wound coilsare alternated. This can enhance the stability of the disc replacement,and can prevent overrotation. FIG. 17 illustrates a disc replacementhaving a first plurality of column springs disposed below a secondplurality of column springs. As shown in FIG. 17, the lower coils can bewound in the same direction as their upper counterparts, or can be wouldin the opposite direction.

FIG. 18 shows a single compressible column spring according to someembodiments. When maximally compressed, the spring makes a continuouscolumn. However, as described above, the embodiments of the inventionare not limited to column springs having rectangular cross-sections likethe one shown in FIG. 18. The edges of the coils may be shaved orrounded such that, when maximally compressed, the spring forms a columnwhose outer surface is uneven, different from that of a straightcylinder.

An intervertebral disc replacement according to some additionalembodiments is shown in FIG. 19. The intervertebral disc replacement hasa second layer comprising a compressible lattice. FIG. 20 shows azoomed-in image of the compressible lattice according to someembodiments. The compressible lattice can be formed from layers ofsprings, or “struts.” According to some embodiments, each strut 2000 hasan O-like shape that can be compressed, thereby reducing the height ofthe strut, and can expand and/or return to its original state dependingupon the forces being applied. For example, the struts can be of ashape-memory type so that when the compressive forces are removed, thestruts return to their original position. Each strut can furthercomprise a stopper 2002. The stopper 2002 can be any mechanism thatlimits the extent to which the strut 2000 can be compressed, therebypreventing mechanical failure of the strut 100 due to excessivecompression. The stoppers thus provide an elastic limit so that thestruts do not fatigue or fracture. According to some embodiments, thestopper 2002 comprises two T-like features, an upright T-like feature2004, and an inverted T-like feature 2006, disposed within the O-likestrut 2000. When the strut 2000 is compressed, the T-like features 2004,2006 are brought nearer to each other. The compression continues untilthe two T-like features 2004, 2006 come into contact with one another.The features prevent the strut from further compressing, providing aminimum displacement between the top and bottom layers of the discreplacement. The elasticity and/or shape memory of the struts can allowthe device to accommodate expansion and compression. Advantageously, thelattice structure shown in the figures undergoes almost no lateralexpansion when the disc replacement is compressed vertically.

According to some embodiments, the compressive quality of the latticecan be a result of the compressive design of each of the struts. Thecompressive lattice can be finely tuned to mimic the compressivecharacteristics of a native disc. For example, certain areas of thecompressive lattice can be designed to be more or less resistant tocompression, essentially changing the spring constant. One can thusdesign regions of the disc replacement known to experience greaterforces to be less compliant, while less load-bearing areas can be morecompressible.

FIG. 21 shows a side view of an intervertebral disc replacementcomprising a compressible lattice. FIG. 22 shows a cross-section of anintervertebral disc replacement comprising a compressible lattice. FIG.23 shows the first and second layers of an intervertebral discreplacement comprising a compressible lattice. FIG. 24 shows a top viewof the compressible lattice. According to some embodiments, the latticehas a repeating hexagonal structure. While the lattices shown in thefigures have a regular, repeating structure, the shape and size of theindividual elements can in fact be varied. For example, in some regionsthe lattice can be thicker to provide additional support and resistanceto compression. The lattice is also not limited to the number of layersshown in the figures. The lattice could comprise more or fewer layers,and the struts could be made larger or smaller. For the latticecomprising column springs, the springs within the lattice may havedifferent diameters and thicknesses, and the spacing between them mayvary. These parameters may be adjusted such that different regions havedifferent compressive and rotational qualities, allowing the discreplacement to mimic the movement of a native disc.

According to some embodiments of the invention, the intervertebral discreplacement is fabricated using 3D printing. For example, a powder-based3D printer, such as a powder and laser-based 3D printer, can be used toform the first layer, the second layer comprising a plurality of columnsprings, and the third layer. The disc replacement can be formed as asingle piece, or as multiple pieces that are then connected. Accordingto some embodiments, the column springs are printed directly on top ofthe first layer, and the third layer is printed on top of the pluralityof column springs. Similarly, for embodiments of the disc replacementhaving more than three layers, each layer can be printed directly on thelayer below it. During the formation process, the disc replacement mayalso be inverted such that the layer that will be uppermost in thepatient's body is printed first. According to some embodiments, the discreplacement comprises one or more of titanium, nitinol, cobalt chrome,high density polycarbonate, and any other material used for implants inthe human body. Titanium can be advantageous because it is not magneticor ferromagnetic, and is biocompatible. All biocompatible materials arecontemplated in accordance with the principles of the invention.

According to some embodiments, the 3D printing process includes printingusing multiple materials in a single disc. For example, the first and/orthird layer may be printed using a first material, while the pluralityof column springs is printed using a second material. The column springsthemselves may comprise more than one material. For example, differentmaterials may be used to vary the spring constant within a single columnspring, or from one column spring to the next. Multiple materials mayalso be used to form the first layer and the third layer. The firstlayer may comprise different material(s) than the third layer accordingto some embodiments.

According to some embodiments of the invention, the disc replacement isformed using sintering. Sintering is the process of compacting andforming a solid mass of material by heat and/or pressure without meltingit to the point of liquefaction. However, the embodiments of theinvention are not limited to 3D printing and sintering, and otherappropriate manufacturing methods may also be implemented.

Because the device can be 3D printed, its shape, as well as itscompressive and rotational characteristics, can be patient-specific. Forexample, the surface area of the first and third layers may appearelliptical, kidney shaped, oval, or trapezoidal from a top-down view,while appearing scalloped or winged from a cross-sectional view, forexample. The dimensions of the disc replacement are patient-specific,designed to match the size of the native disc, and meet the rotational,compressive, and load-bearing requirements of the native disc that willbe replaced.

The design of the disc replacement may take into account data from anX-ray, a magnetic resonance image (MRI), or a computer tomography (CT)scan. The disc replacement may also be tailored in a patient specificmanner taking into account the exact patient disc geometry fromX-ray/Mill/CT, the disc level (each disc level requires differentmechanical properties), the weight of the patient body portion above thedisc, etc. Based on these factors, the disc replacement will be apatient specific disc in which the end plates and their angle, the discreplacement's volume, its springs geometry (both the individualconstruction of each spring and its resistance, as well as number ofsprings and their disposition along the disc area) are all tailored tomeet the exact needs of the patient.

The embodiments illustrated and discussed in this specification areintended only to teach those skilled in the art how to make and use theinvention. In describing embodiments of the invention, specificterminology is employed for the sake of clarity. However, the inventionis not intended to be limited to the specific terminology so selected.The above-described embodiments of the invention may be modified orvaried, without departing from the invention, as appreciated by thoseskilled in the art in light of the above teachings. It is therefore tobe understood that, within the scope of the claims and theirequivalents, the invention may be practiced otherwise than asspecifically described. Moreover, features described in connection withone embodiment of the invention may be used in conjunction with otherembodiments, even if not explicitly stated above.

We claim:
 1. An intervertebral disc replacement comprising: a firstlayer having a lower surface for contacting a first vertebral bone; asecond layer coupled to the first layer, the second layer comprising aplurality of compressible column springs; and a third layer coupled tothe second layer, the third layer having an upper surface for contactinga second vertebral bone, wherein each of the plurality of compressiblecolumn springs comprises a plurality of stacked coils, wherein each coilof each of the plurality of stacked coils has a spring constant (K), andwherein at least one of the plurality of compressible column springscomprises a first coil having a first spring constant and a second coilcomprising a second spring constant, wherein the first spring constantis different from the second spring constant.
 2. The intervertebral discreplacement of claim 1, wherein at least a second compressible columnspring of the plurality of compressible column springs comprises a coilhaving a third spring constant, wherein the third spring constant isdifferent from at least one of the first and second spring constants. 3.The intervertebral disc replacement according to claim 1, wherein thedisc replacement undergoes compression at a first rate when acompressive force is applied until a height of the disc replacementreaches a first predetermined value, and wherein the disc replacementundergoes compression at a second rate as the height of the discreplacement is further reduced, wherein the second rate is less than thefirst rate.
 4. The intervertebral disc replacement according to claim 1,wherein each of the first layer and the third layer has a plurality offixation mechanisms that engage the first vertebral bone or the secondvertebral bone to fix the position of the intervertebral discreplacement with respect to the first and second vertebral bones.
 5. Theintervertebral disc replacement according to claim 1, wherein each coilhas a cross-section, wherein the cross-section of each coil has a flatlower surface and a flat upper surface that is parallel to the flatlower surface.
 6. The intervertebral disc replacement according to claim5, wherein each cross-section has a maximum width, and wherein each ofthe flat upper surface and the flat lower surface has a width that is atleast 30% of the maximum width.
 7. The intervertebral disc replacementof claim 5, wherein each cross-section has a maximum width, and whereineach of the flat upper surface and the flat lower surface has a widththat is at least 90% of the maximum width.
 8. The intervertebral discreplacement of claim 5, wherein each cross-section is substantiallyrectangular.
 9. The intervertebral disc replacement of claim 1, whereinthe plurality of compressible column springs are arranged in one or moreconcentric rings around the center of motion of the intervertebral discreplacement.
 10. The intervertebral disc replacement of claim 1, whereinat least one of the plurality of compressible column springs comprisescoils that are wound clockwise, and wherein at least one of theplurality of compressible column springs comprises coils that are woundcounter-clockwise.
 11. The intervertebral disc replacement of claim 1,wherein the intervertebral disc replacement mimics the compressive,extension, flexion, and rotational behavior of a human intervertebraldisc.
 12. The intervertebral disc replacement of claim 1, furthercomprising a barrier attached to the first layer and the third layer andenclosing the second layer, wherein the barrier forms a fluid-tight sealwith the first layer and the third layer.
 13. The intervertebral discreplacement of claim 1, wherein each column spring has an outercircumference between about 3 mm and about 40 mm.
 14. The intervertebraldisc replacement according to claim 1, wherein the intervertebral discreplacement comprises one or more of titanium, nitinol, cobalt chrome,and high density polycarbonate.
 15. The intervertebral disc replacementaccording to claim 1, wherein each of the plurality of compressiblecolumn springs has a height that decreases when a compressive force isapplied, and a structure that prevents the height from decreasing beyonda predetermined limit.
 16. The intervertebral disc replacement accordingto claim 1, wherein a difference between the first spring constant andthe second spring constant is determined by a difference in at least oneof a group consisting of a height of a cross section, a width of across-section, a pitch, and a material of the first coil and the secondcoil.
 17. The intervertebral disc replacement according to claim 1,wherein the first layer, the second layer, and the third layer are oneunitary piece.
 18. The intervertebral disc replacement according toclaim 1, wherein the second layer comprises: a first plurality ofcompressible column springs attached to the upper surface of the firstlayer; a second plurality of compressible column springs attached to thelower surface of the third layer; and a fourth layer disposed betweenand attached to the first plurality of compressible column springs andthe second plurality of compressible column springs.
 19. Theintervertebral disc replacement according to claim 1, wherein one orboth of the first layer and the third layer has a height that varies.20. A method of producing an intervertebral disc replacement, the methodcomprising: forming, using a powder-based 3D printer, a first layer;forming, using the powder-based 3D printer, a second layer on top of thefirst layer, the second layer comprising a plurality of compressiblecolumn springs; forming, using the powder-based 3D printer, a thirdlayer on top of the second layer; and removing unbound powder from theplurality of compressible column springs, wherein each of the pluralityof compressible column springs comprises a plurality of stacked coils,wherein each coil of each of the plurality of stacked coils has a springconstant, and wherein at least one of the plurality of compressiblecolumn springs is formed to include a first coil having a first springconstant and a second coil having a second spring constant, wherein thefirst spring constant is different from the second spring constant. 21.The method of producing an intervertebral disc replacement according to20, further comprising: prior to forming the intervertebral discreplacement, obtaining at least one of x-ray, magnetic resonance imaging(MRI), computed tomography (CT), patient body mass above the discreplacement; and determining the angles of the end plates and theposition of each of the plurality of compressible column springs and thespring constants of each of the plurality of stacked coils based on theobtained data to match patient specific level lordosis and movementneeds.
 22. A method of producing an intervertebral disc replacementaccording to 20, where forming the second layer comprises: forming,using the powder-based 3D printer, a first plurality of compressiblecolumn springs attached to the upper surface of the first layer;forming, using the powder-based 3D printer, a fourth layer attached tothe first plurality of compressible column springs; and forming, usingthe powder-based 3D printer, a second plurality of compressible columnsprings attached to the fourth layer and the lower surface of the thirdlayer.